When first turned on, many electronic devices consume a transient peak current, which can cause voltage fluctuations and affect the performance of other circuits connected to a common power supply. The transient peak current is sometimes referred to as an inrush current. In switching power regulators, including those with bridge type power stages, charging of gate capacitances, bridge capacitances, and reactive loads can introduce these transient peak currents during startup, stressing or damaging associated circuitry, if such peak currents are not adequately controlled. Generally, switching power regulators include soft-start circuitry configured to suppress the transient peak currents by controlling the charging rates of any reactive elements.
One technique for controlling such peak currents includes biasing the switching field effect transistors (FETs) into their linear region. Controlling the FETS in this manner reduces the peak currents, but applies undesirable stress to the FETs. Further, in this technique, the switching power supply does not operate properly at higher power levels without expensive, high power FETs.
Another technique for controlling peak currents uses a frequency ramp soft-start method where the frequency of drive pulses for controlling the FETs decreases over time from a high frequency down to an operating frequency level. Unfortunately, this technique still introduces a large current spike on start up that is dependent on the size of the resonant capacitor and on the charge stored by the resonant capacitor on startup. If the resonant capacitor is large and/or there is no stored charge, the current spike will also be large as the resonant capacitor charges.
Still another technique for controlling such peak currents involves increasing the amplitudes of the gate drive signals of the switching FETs. In this method, the gate voltages of the high-side and low-side FETs are slowly increased with each cycle. The low-gate voltage for each turn on cycle makes the FETs more resistive and slowly ramps the inductor current. However, cyclically increasing the gate amplitudes can cause overdissipation on the FETs and reliability issues and may not work in some applications.